Production of metal powder



Mgy 19, 1953 H. A GOLWYNNE PRODUCTION oF METAL POWDER 5 Sheets- Sheet 1Filed Sept. 29, 1949 wm @k INVENTOR.

HENRY A. GOLWYNNE 'BY 17 '2, FTamwm ATTORNEYS N@ ww May 19, 1953 H. A.GOLWYNNE 2,638,630

PRODUCTION oF METAL POWDER Filed sept. 29, 1949 .5 srie'c-sheet 2ATTORNEYS May 19, 1953 H. A. GOLWYNNE PRODUCTION OF METAL POWDER 5Sheets-Sheet 3 Filed sept.- 29. 194s INVENTOR. HENRY A. GOLWYNNEmffmmmwmmw aw! Hull ATTORNEYS May 19, 1953 H, A. GoLwYNNE PRODUCTION oFMETALv POWDER 5v Sheets-Sheet 4 Filed Sept. 29. 1949 'mvo A QN INVENTOR.

HENRY A. GoLwYNNE ATTORNEYS May 19;'1953 H. A. GoLwYNNE A2,638,630

PRoDUcTIoN oF METAL POWDER Filed Sept. 29AI 1949 5 Sheets-Sheet 5INVENTOR. HENRY A. GLWYNNE BY 'EM/m, fammi/fm Pannmm ATTORNEYS PatentedMay 19, 1953 UNITED @PATENT o rfi-ICE "PRODUCTION "DF METAL yPOWDERHenryuolwynne, New York, N. Y.

Allplcaatitm September 29, 1949, Serial No.'11'8,621

20 Claims. -1

This invention relates 'to vthe production of metal powders and hasforits object improvements inthe methodof producingmetalpowder.,Amongthe methods of producing'metal; powder is that of atomizing'moltenmetal with ahlast of gas, freezing the *atomized 'metal-intofnelydivided solid particles,'and'recovering the resulting metal powder.'Most metals'have a 'marked kafiinity "for `.oxygen and nitrogen atelevated temperatures. metal powder from molten metal'byatomization witha gas because'the 'oxides and nitrides'formed b5/'the reaction of oxygenandnitrogenin'the gas with the atomizedVA metal tend'to vform at "ornear and to contact aniithen-to'adhere'to the highly heatedatomizing'nozzle*thus clogging or otherwise impairingiitsusefulness`Suchnotzles usually have a central outlet or discharge openin'throughWhichthemolten'metal is 'forced and usually aiplurality ofports throughwhich; streams of thegasare forced against the 'molten metal dischargedfrom the outlet. Although a igreat deal of the oxide and nitride findsitsvvay into the vmetal vpowderproduced`y appreciable amounts contactthe hot nozzle-and adhereltheretoasian Yaccretion orincrustation. 'Thisbuilds up around the ports, particularly, and even-the outlet,eventually, -so that'theybecoine smaller and smaller and cloggedeventually. Asthis takes place'the size of the molten metalandgasstreamsisgrad- ,l

ually reduced, thus altering radically the conditions 4under -which thepowder is Iformed. This aiects theparticle size-ofthe product as well'as'the eciency of theoperation. The nozzles Imust 'be removed, repaired-or 'replaced fvrequernly.l

In the earlier attempts toprodnce'metallpowder by atomization, thegasemployed'for atomization andin the atomizing-chamber vwas air orsteam. 4Air is highly oxidizing and nitrid-ing andstearn mildlyoxidizing. lThey vhave been `discarded favor of sc-called inertornon-reacting gases, As we shall seeinertgases available commercially inquantities are not-truly inert,and\even-fi truly inert ether oxidizingandnitriding influences are inherent inthe operation. Inanyeventvariousg proposals'haveloeen advanced to atomize molten metal with `and in 4thepresence of such inert f or non-reactiveatmospheres. For example,aatrickle Orstrearn-of molten-metal iss-blasted in'aeharnt-v bei; filledwith inert gasythefatomized -metalvis f rozen linto 'nely divided solidparticles and the resulting metal powder permitted to sett-leiat-thfevbottom of `the chamber jfrom l which it isremoved periodically.Another-proposalis to spraymolten magnesium intoa- 'ohamberilled withcold Initro- This makes it diflicult to 'produce riods.

gen gas deemed to be comparatively inert to the magnesium. Magnesium,however, has a high affinity i or nitrogen, when either or both isheated, and itis'impossible to spray molten magnesium, which isnecessarily highly heated, insuch a man- Iier W-ithout providing a zone,even though restricted in extent, conducive to the formation of anobjectionable amount of magnesium nitride. As before the powder isremoved periodically from the'bottom of the chamber.

Investigators in this el'd have been greatly troubled with the problemof producing a suinciently inert or non-reactive atmosphere in which toatomize the molten metal. As manufactured, the gas itself, for examplehelium, argon, etc., is not entirely inertbecause it usually contains acertain amount of impurities such as oxygen and. nitrogen. The inert gasmust'be conducted into the chamber in which the molten metal is to beatomized and the powder formed. Since the 'chamberis initially lled withair, the air'n'iust be flushed out, or otherwise be'replaced, by theinert gas. `Sncegases, mix readily, the inert gas and-air promptlyinterni-ix. Inv spite of careful `precautions a residuum of air remains,and since the air isrich in :oxygen and nitrogen, the atmosphere as aWhole in the chamber isfcontami- Ihated to that extent.

The atomized metal reacts readily with the oxygen and nitrogen. vOxideand nitride incrustations thenforin on the nozzle -used toatomize themolten metal.

The discharge ports of the nozaflev are soon cloggedand the operationmust Vloe stopped to clean or replace the nozzle.

--method of producing metal powder by atomizing molten metal witha:blast of'inert gas, metal powders may be 'produced steadily overprolongedV pe- 'Meta'l powders substantially uniform in quality land inyparticle size may be obtained. The operation maybe so conducted as toobtain metal `powders graded generally as to particle 4Inaccordance withoneaspect of the-invention, a confined circulatory system-includingmolten metal atomizing, metal powder forming and powderseparatingzzones-is.lled initially with inert gas under pressurehigherthan atmospheric to prevent. ingress ofvoutsideair. rilhe inert gas inthe .system is puried with respect to such impuritiesasrvoxygenandnitrogen, after which-nieta 'powder is produced therein.

Air initially present in the circulatory system is replaced for the mostpart with inert gas. The inert gas is introduced in the system at a highpoint and the undesired air containing oxygen and nitrogen is withdrawnat a low point. The oxygen and nitrogen in the inert gas and in theresidue of air remaining with the inert gas in the system are theneliminated.

The purification operation is advantageously conducted by causing theoxygen and nitrogen impurities in the gas to react with heated metalparticles to form the oxide and nitride 'of the metal. Metal powder ofthe kind to be produced, or previously produced, is preferably employedbecause it is at hand. The gas may be withdrawn temporarily from thesystem, purified, and then returned to the system; the entire operationbeing a circulatory or cyclical 'one until all of the gas in the systemhas been in Contact with the heated metal for puriiication and has beenreturned to the system.

According to another aspect of the invention, the molten metalatomizing, metal powder forming and powder separating steps areconducted in the confined circulatory system filled with the purifiedinert gas as the gas is continuously circulated therethrough underpressure vhigher than atmospheric to prevent ingress of outside air.This is advantageously accomplished by blasting a nne stream of themolten metal circumferentially with a plurality of streams of heatedinert gas in the metal powder forming zone filled with the circulatinginert gas, the circulating gas being sufiiciently low in temperaturepromptly to freeze the atomized metal into nely divided particles. Thecirculating inert gas with metal powder suspended therein is passed fromthe powder forming zone to a powder-gas separating zone; metal powder isseparated from the inert gas; the inert gas remains in the system tobe'reused; and the metal powder so separated is withdrawn from thesystem.

To Iobtain the plurality of iine streams of inert gas to atomize themolten metal, it is advantageous to withdraw temporarily some of theinert gas from the systemy place it under substantial positive pressure,and return it to the system by atomizing the molten metal. The gas sowithdrawn from the system is preferably preheated by placing it inheat-interchange relation- I ship with the body of molten metal to beatomized. The fine streams of heated inert gas are then forced underhigh pressure into the streamrof molten metal thus breaking it up into amyriad of fine droplets which are then enveloped in the cooler inert gascirculating through the system and frozen into solid particles. If coolgas were used to atomize the molten metal, at least in the atomizersconventionally employed, molten metal would tend to freeze and adhere tothe discharge tip.

Another highly advantageous expedient is to place the source or body ofmolten metal to be atomized under substantial pressure with the inertgas so that molten metal may be forced as a stream into the atomizingzone. To this end some of the inert gas is withdrawn from the system,conducted lunder substantial positive `pressure to a confined spaceabove the body of molten metal and used to force a stream of the moltenmetal to the atomizing zone.

In a presently preferred practice of the invention, the circulatinginert gas with metal powder suspended therein is passed from the powderforming zone through a plurality of powder-gas separating zones, metalpowder being separated from the inert gas in each zone and the metalpowder so separated is withdrawn from the system at each zone.

The separation of the metal powder from the gas may be conducted invarious ways. At the present time the gas with metal powder suspendedtherein is passed successively through a series of -cyclones so that theseparation of the powder may take place by dry precipitation. This hasthe advantage that powder of graded particle size may be selectivelyseparated in each cyclone; for example, ne particles in the iirstcyclone, finer particles in the next cyclone, and so on. The residue ofdust remaining in the gas leaving the last cyclone is advantageouslythrown down by wet precipitation, suc'h as by passing the gas throughabag lter wet with oil. As an additional precaution it is desirable touse a filter in the line going to the compressor.

As already indicated, improvements in the apparatus employed arenecessary to obtain the advantages of the method. The apparatus forproducing the metal powder comprises a conned circulatory system adaptedto be filled with gas under pressure higher than atmospheric; agasometer to hold surplus gas and to take care of expansion andcontraction of the gas; a chamber for atomizing molten metal andchilling atomized metal into powder; a -blower for circulating the gasaround the system; and a separator for separating the powder from thegas.

An atomizer for the molten metal extends at its discharge end into thechamber. The atomizer in turn connects with the bottom portion of amelting furnace and means are provided for exerting pressure inside thefurnace so that molten metal at the bottom may be forced through theatomizer into the chamber. This pressure is preferably obtained with acompressor connected on the inlet side with the system for thewithdrawal of some gas and on the outlet side with the interior of themelting furnace to place a layer of gas over the body of molten metalunder sufcient pressure to force molten metal through the atomizer. Thecompressor preferably connects on the inlet side with the system beyondthe place or places where the powder is separated from the gas so thatsubstantially powder-free .gas may be passed by the compressor to thefurnace.

Another advantageous arrangement is one adapted to use some of the gasin the system for atomizing the molten-metal. To this end thecompressor, as before, connects on the inlet side with the system forthe withdrawal of some gas and on the outlet side with a conduit inheatinterchange relationship with the furnace for melting the metal tobe atomized, the discharge end of the conduit connecting with theatomizer for the molten metal so that the lmolten metal discharged fromthe atomizer may be blasted with gas preheated by the furnace. Also, asjust described above for the furnace, the compressor preferably connectson the inlet side with the system 4beyond the place or places where thepowder is separated from the gas so that substantially powder-free gasmay be passed bythe compressor through the preheating conduit to theatomizer for atomizing the moltenmetal.

A combination of the features just described is highly desirable inpractice. In other words, the compressor connects on its outlet sidewith the interior of the furnace as Well as with the atomizer, so thatmolten metal in the furnace may .be placed underpressure :by the as.aand thus be forced Afrom the furnace` .toiandthroueh the atomizerandsothat thefinolten metal may be atomized by the gas from the compressorwhen itisy dischargedinto therchamber. The compressor .connects on theinlet Side with the System beyond the place where the powder:isseparated from the gas .so vthat substantially powder-.free gas maybepassed by the compressor to; the fur nace and to the .atomizer.

The invention is .adapted for the .production of a variety of-metalpowders. The. metal to be converted into powderinay.be'essentially aDrimary metal -or a combination :of metals, `such as-alloys. As apracticalimatter .one-,of `.thedetermining factors-is, ofcourse, .themelting point of the metal. Thehigher `the melting point, .the morearethe diiiicultiesI encountered;-.partieularly as regards heating andwear vand `tear of the equipment. The invention .is .being .currentlypracticed in .the productionv otmagnesiurnpowder, and powders of alloysof magnesium, more particularly alloysof magnesium andfaluminum.

While various gases inert to thametal to be converted to powder may beused-asa practical matter the choice sinimers .down toone readilyavailable at a relativelyilow cost. Forthis reason helium is now beingused. Otherfinertgases, such as argon could,of course, beernployed.

The practice .of the ,invent-ion as described results inanunusuallystable and .eicient product. The .metal powder may be .storedwithout appearingtolose its eiectveness inzuse. In the case ofmagnesium, the .powder .remains highly effective after storage. .This isnot true of conventional magnesiumpow'ders such as .are produced bygrinding or attritional means. This may be due to the fact that suchpowders are produced in a relatively hot State due to the grinding orattritionalifrction, which vcausesthe exposed-surfaces of theparticles'to react'with the surrounding gases and thus,"for .eXampleftotake on a coating'iof oxide. 'On the otherhand powder 'of kthe presentinvention 'is produced in a relatively cool atmosphere due to the largevolumes of cool circulating inert gas=that envelop and chill theparticlespromptly on formation, and carry them awaytfrom the atomizing`and powder forming'zones. There-islittle or no opportunity for anoxide'coating to form --on-the individual particles.

linl addition the metal powder 4of the invention is highly `activatedthusmalrin-g lit -Very eiiicient in use. rThis appears tolbedue-totheenormous amount of surface 'area-offered by-a body of theproduct, far in lexcess of that lcaf-conventional metal powders. Metalvpowders as heretofore made aresolid andltherefore expose only anexterior surface. The powder-of'theinvention, on the other hand, is inthe vform ofy hollow spherical particles. The interior surfacesandreXteriOr surfaces together provide an .enormous -amount of availablesurface for use. This is particularly important, for; example, in.theucase of magnesium powder used for flares. .Due to 'the .enormousamount` of available surface areaithelmagnesium powdermay be .burnedinahighly intense instantaneous'. flash.

.Itis at present believed .that-.the particles are formedvv as theresultoi the marmer viirvrlu'on the inert gases .are employed. .As notedabove, .a portion'of the ineringas is .withdrawn-vfromithe circulatorysystem and conductedto and over .the body of mol-ten metalzinfthefurnaceunderrsub- .stantialpositivepressure; in .order to. force moltenmetal from the urnace to .theiatomizen result of this step the moltenmetal becomes-'Saburatedzwith Y.the .feas fat :the prevailing[temperalturciand :pressure :in fthe :melting iurnace, Qausinga;.certain=.-a1nount.zoffthe.: sas tof go intofsolution in ithe.. moltenimetal. 'When v.the molten metall is .fatomized .the resulting-:minutedroplets arezpromptlyfrozenyby .the cool circulating inert gaszintosolid g: particles. .The chilling z eitect s isv so rapid 'that the gas:dissolved `the drnlets quickly :forced aoutzof. solution thus'. causingthem toxexpandintofparticles.

.Theseiand- :otheryfeatures'ofzthe invention yWill be .better.undeistoodfhy referring :to the accomparuringfdrawings, taken .in.conjunction-.With the following; descriptiomin which:

:l is fa :diagrammatic flow sheet showing' the system asa whole;

Eig .2 .isa side elevation of the .purifier `shown in:Fig.f1;

.Eig. `Sgislafplan'view offene of the=trays in the purifier;

iFig. f4 :is a cross-section .zon ,theline "L -e4 of Fig;

x5 isa crosse-sectionalviewof Aone .ofthe melting urnacesishown inFig..1;-

iFig. .6 iis 'anenlarged cross-sectional View .of the atomizer shownin:;Eig.:.5;

Tig. 7 a? longitudinalsection iofithe chamber shouminilfig. 1 :foratomizingmolten :metal and forming :metalv powder;

Figt iszatransverse ksectionyon line --.e .of Eign;

;Fig,.9 ..is an .enlarged Vside viewrof :one of `the powder-:gastseparatorsf'showrrinlllig.'1;

Fig. ,10 is a similar .vieiv, .partly in section, of the f lowerend-thereof; ...and

.Fig 11 is;a front elevation ofrtheesame.

Reference is made lto Fig. il for ran overall picture :of the:apparatus. Since .'pnriiication of the .gas in `the system -isiofinitial importance, the ymain featuresfof rits sy-stein :or circuit willrst bezdescribed.

iincludes a .gasometer1i,.a conduit 2,.a main Vconduit f3, a branchconduit-Ara dustriilter 5, :a yconduit P16, a brancheondzui-t f1,aslter'ua branch conduit :-;9,.a.brar1ch conduit Ilka lter IJ, :faphi-'anch .conduit 12, .a :compressor I3 .cone nected on .its .inlet or:suction :side with conduit S,:.af,conduit vtLccnneeting the :outletorpressure side of the compressor, a filter I5, afconduit .113, a branchconduit :1:1reonnectingztheLinletof a purifier :t8 surrounded by aheating arurnace or chamber l li9 Lhaving :an rinlet 2 0. rfor the.introductionof heating :gasesfand fan outlet 2l .for y.the escape ofspent gasesgandaconduit 2.2connectingnthe outleto'f the'purier-withzthegasorneter. The latter oonduitrmayfof course, .connect the systematany.v other .suitable point. mail@ some valves enclosure :members areindicated, `it .will -be clear-"that any suitable lnumber may.beeinployedat other pointsin the circuit. ln general, it will-beseenftha-tf-gas inayipass-f-rcm'themain systemby vway4 of thevgasoineterl and main conduit 3 to -and'through vdust lter5,one 'orbothfilters 8 Vand I. l to and" through the ycorrrpressor, through filter"f5, purifier "I8 and backinto the system. "This cyclic operation maybecontinued until all of .the gas in Vthe .system reaches .the desiredpurity. What has been'described may be ,regarded aspart .of the Kmainsystem, .o1-'indeed acirculating. system within themaincirculatingfsystem. When. all .of thegas` is purified, .it is yunnecessary `.to...continue with. .the ,purification operation and itmay be shut-off until again needed.

The main system or circuit for producing the metal powder will now betraced. It includes a branch conduit 30 connecting main conduit 3 with abonnet 3l at one end of a molten metal atomizing and powder formingchamber 32 for providing cool inert gas with which to freeze theatomized metal to powder. Another branch conduit 33 also connects mainconduit 3 with the lower portion of the chamber in order to supplyadditional gas for keeping the newly formed metal powder in suspension,as will be described in more detail below. A branch conduit 34 similarto branch conduit 30, connects main conduit 3 with a bonnet 35 at theother end of the chamber. Conduit 36 connects the same or discharge endof the chamber with a blower 31, a conduit 38, a powder-gas separator40, a conduit 4|, another powder-gas separator 42, a conduit 43, and afilter 44 which in turn connects main conduit 3. A branch conduit 45connects the main conduit with a filter 46 and a conduit 41. Similarly abranch conduit 50 connects the main conduit with a filter and conduit52. In practice it is customary to alternate use of filters 46 and 5|,so that while one is being cleaned or otherwise serviced, the other isused.

'Ihe next system or circuit to be traced is the one having to do withthe use of gas to force a fine stream of molten metal to the atomizerand the use of gas to atomize the stream of molten metal. It includesconduit I6, from the outlet or pressure side of compressor I3 and abranch conduit 60 terminating in branch conduits 6| and 62. Branch 6|connects with a conduit 63 one end of which connects with the interiorof a melting furnace 64 and the other end of which connects with theinterior of chamber 32. A bonnet 65 connects the furnace to bonnet 3| ofthe atomizing and powder forming chamber.

A similar arrangement is provided for the other end of the chamber andincludes a branch conduit connecting conduit I6 and terminating inbranch conduits 1| and 12. Branch 1| connects with a conduit 13 one endof which connects with the interior of a melting furnace 'I4 and theother end of which connects with the interior of chamber 32. A bonnetconnects the furnace to bonnet 35 of the atomizing and powder formingchamber.

Figs. 2, 3 and 4 show the purier in detail. It is cylindrical in formwith a closed bottom 80 (see Fig. 2) resting on supports 8| on thebottom of the interior of the heating chamber. Superposed trays orbaskets 82 rest on an inner flange support 83 integrally secured to theside wall of the purifier, the flange being located directly above inletI1. An imperforate cover 85 is securable to the top of the purifier bymeans of a plurality of bolts 36 through an outer flange 81. All jointsof the purifier are gas tight so that heating chamber gases cannot enterthe purier and so that gases inside the purier cannot escape into theheating chamber.

The trays (see Figs. 3 and 4) are generally cylindrical in shape, beingformed of a band of sheet metal 90 with a turned in bottom flange 9| anda similar turned in top flange 92. A circular piece of coarse meshcircular screen 94 rests on and is secured to the bottom flange. Acircular piece of fine mesh screen 95 in turn rests on the coarse meshscreen, the latter serving as a firm support for the former. Arelatively thin layer of finely divided metal particles 96,

y8 such as magnesium powder, if magnesium powder is to be produced,rests on the screen. A handle or bar 91 is secured to the underside ofupper flange 92.

In preparing the purifier for use, the top of heating chamber I9 isremoved, cover 85 of purier I8 is also removed and loaded trays orbaskets B2are placed therein, one superposed on the other. In the caseof the bottom tray, its bottom flange 9| rests on inner flange support83. Lower flange 9| of the second tray will rest on upper iiange 92 ofthe rst tray, etc. The purier is provided with a suiiicient number ofthe loaded trays. Cover is then returned and bolted securely to outerflange B1 with bolts 86; after which the top of the purifier isreturned.

Melting furnace 64 is shown in detail in Fig. 5. It rests on a dolly sothat it may be wheeled into and away from operating position withrespect to chamber 32. The furnace is formed of a rectangular heatingchamber 9| surrounded by a metal casing 92 lined with refractory brick93. The chamber is surmounted by a removable top 95. A plurality ofspaced openings 91 are provided through the side walls near the bottomof the chamber into each of which is fitted an oil or gas burner 98.Three such burners are at present employed.

A stack 99 is mounted on the top for the escape of spent heating gases.A melting pot |00 rests on supports |0| at the bottom of the heatingchamber. The top of the melting pot is provided with a removable cover|02 securable to an outer flange |03 by means of a plurality of bolts|04 to provide a non-leaking joint. A metal charging conduit |06 extendsthrough the top of the heating chamber and communicates with theinterior of the melting pot. It is secured to cover |02 by means of aplurality of bolts |01. A removable cover ||0 fits over the top of thecharging conduit and can be screwed thereon to make a leak-proof joint.Branch conduit 93 for inert gas connects with the charging conduit abovefurnace top 95. A pyrometer ||2 extends through the top and cover |02well into the interior of the melting pot. Branch conduit 62 for inertgas extends into heating chamber 9| and is spirally wound around themelting pot. The discharging end of the conduit extends into theinterior passageway lill of bricklined furnace bonnet 65. A conduit ||5for the passage of molten metal extends from near the bottom of themelting pot upwardly through its cover |02 and into passageway ||4 offurnace bonnet 65 where it connects with an atomizer H6 extendingthrough end-cover ||1 into chamber bonnet 3|. The two bonnets areconnectible with bolts ||9. The furnace bonnet is provided with anopening |20 in close proximity to the atomizer and is tted with a burner|22 adapted to supply heating gases at and around the atomizer.

Atomizer l I6 is shown in'more detail in Fig. 6. It is in a form of amain body portion |24 hollowed out at one end to receive the dischargeend of conduit I5 from the melting pot and hollowed out at the other endto receive an atomizing nozzle |25 and to form a gas distributingchamber |26 around the nozzle. The tip end of the nozzle, with dischargeoutlet |28, extends through cap |29 screwed onto the other end of themain body portion. Conduit 62 for inert gas connects with a nipple |30communicating with the gas distributing chamber. Cap |29 9,-' is:provided with a plurality ofl inclinedY circumferentially spaced"discharge ports |32 adapted to blast small `streams of preheated`inert-gas'against a stream oiv molten metal dischargedt through nozzleoutletv lili? at a point. orarea |34a suit;-

able distance forward ofv the nozzle tip.

Figs. 'I and i show chamber 32: for atomizing molten metalandformingpowder therefrom in more detail. tankv dividedlessentially into anuppercompartment |40, Which is in the form offa trough, a lower compartment|41, which is essentially in the form of a duct; and two sidecompartments H52 and |43, which are separated from each other as Well asfrom the other two compartments. All of the compartments extendlongitudinally of the chamber. The upper and lower compartments aretapered in reverse order. Thus, upper compartment lll!! hasits smallestcross-section at the right end of thetank, and its largest cross-sectionat the left end. The lower compartment or gas distributing duct i'i, on.the other hand, has its largest oros-section at the right end, Where thegas is shown to enter through conduit 33, and its smallest crosssectionat the left end.

The compartments are obtained by the use of opposed pairs of overlappingplates to |53, top plates I6! to w8 and a pair of spaced side plates Hi'and W2. They are suitably, welded to the cylindrical walland to eachother to provide an integral structure. Plates |53 to |58 are pitched orslanted atan angle, toward'the bottom of the trough, so that powdersettling thereon will tend to slide down into the bottom or" the trough.Referring for a moment to Fig. 7, it will be seen that an overlying lipVM is welded to the head of the cylindrical tank at.. the right end toprovide a slot '|15l between inlet conduit 33 and upper. compartment|40. Similar underlying lips |75 to IzBZ provide slots |85 to |9i. Upperplate |65 ofythe duct is spacedV above the bottoni of the tank toprovide a `similar slot |92 at the left end of the tank, in directcommunication with outlet conduit 3E. The overlying and underlying lipsare sufficiently longto cause gas passing therethrough to sweep alongthe bottom of the upper compartment. In other words, gas going throughslot |75 sweeps along the top of plate |68, gas passing through slot |85sweeps across the top of plate |51, gas passing through slot |865 sweepsacross the top of plate. |66, etc. A gas blow-oir |95 is provided alongthe. top of the tank. "in case of aneaplosion, the blowoi is opened torelease explosive forces, thus saving the chamber from damage. An inlet|98 connects each side compartment with inlet con.- duit 33. An outletI99'connects each side compartment with outlet 3.6. While the side.compartments are normally sealed from each other andthe othercompartments, it is possible for leaks to develop at the welded joints.l't is therefore desirable to have an arangenient which Wi'li permit theside compartments to be cleared oi air and iilled with inert gas, andthis can be done with the inlets and outlets referred to.

Figs, 9, 1G and 1lv show powder-gas separator M4' 'in some detail.Powder-gas separator i2 is advantageously oi the 'same generalconstruction. Referring rst to, Fig. 9, the apparatus shown is in theform 4of a combination powder-gas separator and collector, ybeingspeciiically a .conventional -cyclone Zdlivith an inlet 2.0i for.powdergas :and any outlet 262 for gas, .and `a .hopper '2263 integrallysecured `to and depending from the It is in the forrnof` a cylindricalcyclone. The lowerendothehopper terminates in a laterally extendingdischarge conduit 205. ator near the .outletv end of which' is anupright relier"A chamber 296;. A, closure member 2M is pivotallysup-ported atrthe outlet.

Referring next to Fig. l0,` i-t` will be seen that relief-chamber. 2%provides `a substantial amount of 'free space-2m1- at its upperendfwhichextends 'a suitable distanceabove the'place where the chamberjointsl the conduit.. The top of the relief chamber is iittedwithascrew.` cap 2| provided withv a plurality of circumferentially spacedhandles 2 2.

ln. the specific construction shown they discharge-ou-tlet a convenient.angle fto cooperate with closure member 201:, which is in the form ofan enclosed chute pivotally or hingedly con.- nected at 2i3fto a fixedsupport 2M.' The chute l'rasa` bottom 2.152 back 2|6; side Walls 2H' and213, -a top Zillancit''a` discharge opening 22|). A.

piece off resilient gasket material 21212., such as rubber, isvintegrally secured to the bottom ofthe chute-so-that it maybe-broughtinto sealingcontactiwitli the dischargeoutlet.

The discharge outlet is in the form ofa. pipe welded at its sidevtothemain `part of the conduit, the vupper end of the pipe being an extensionwhich forms the relief chamber; The chute is integr ally secured at itsvltop 219 Eto an outer collar 224' iitting looselyv around the lowerendof the discharge conduit. rI'h-e upper'endo thecollar is integrallysecured to the low-er endors, flexible sleeve 225 fitting loosely aroundthe discharge conduit. This sleeve may be or rubberwand is adapted' toact like abellows. The upper` end of the flexible sleeve-is in turnintegrally secu-redvr tothe discharge conduit. When the chute'is Imovedupwardly and downwardly, the lneri-ble sleeve yields sufficientlytoVpermit the" discharge `outlet to be closed and opened.

A handle 228 issecuredfto the top oi the chute at its ldiscl'iargeer1-d` so that the operator may readiiy lower the lchute to 'open thedischarge outlet and raise the chute 'to close the outlet. This is amanuali operation which may remain wholly in control of the operator.

If the operator, however, should -instinctively let. go of' the-handlein case ofire, or for any other reason, or should leave the scene,selfclosing means associated with the chute at once 'take care of suchaccntingency. The particular self-closing means disclosed include aYpair of spring tensioning devicesZ-ll and 23 l" secured at their lowerends to the chuteand at their upper end to a iixedA support, in theinstant construction `at the upper-portion or the relief chamber. A.turnbuckle is attached to each spring so that the springs may be placedunder the proper amount of tension. When downward pressure is applied.lto the chute, such as by bearing Adown on handle 228., the springsyield suiiciently to permi'tthe chute to drop away angularly yfrom thedischarge outlet. To make certain that the chute isno't .lowered unduly,for example, by the operator when excited, a chain 2M of ypredeterminedlength is fastened at its lower end to the chu-te and. at its upper endto a xed support. Theehain fixes and limits the amount of drop for thechute.. device ZBfwithaturnbuckle 238 is hooked Iat its lower end tolthe chute andat its upper end to a fixed support (not shown). When thechute is in its closedposition, thet-urnbuckle is tightened. Pressure onhandle 22%.- will, therefore, not open the chute. This feature isparticularly desirable- As a further precaution, a holdin-g` 11` ifchildren or non-trained operators have access to the apparatus.

When, therefore, it is desired to discharge metal powder from theapparatus, the operator must deliberately loosen turnbuckle 238 in orderto release holding device 236. He then 4applies downward pressure onhandle 223, which causes chute 201 to move or open downwardly. Metalpowder then moves by gravity from hopper 203 through conduit 205 and itsdischarge outlet into the chute, and downwardly through the chute out ofits discharge opening 220 into a container 240.

If all goes well the handle is kept down until the desired amount ofpowder runs into the container. The handle is then lifted by theoperator to close the discharge outlet and the holding device is againreplaced. With the self-closing means in use, however, the operator needmerely release the handle and spring-tensioning devices 233 and 23| willlift the chute into its closed position. As `already indicated thisfeature is particularly desirable in case of fire and if the operatorshould let go of the handle o-r leave the scene in fright. In case offire this is precisely what he should do for safety.

In practicing the method ofthe presen-t invention, the apparatus may beoperated as follows:

Since the circulatory system is initially filled with air, it isimportant that it be replaced with a suitable inert gas, such yashelium. Helium for -this `purpose is obtained commercially in cylinders.With an adequate supply of cylinders on hand, the helium is fed into thesystem at one or more high points and air is removed therefrom at one ormore low points; for example, referring to Fig. 1, the helium may be fedinto gasometer I and air may be withdrawn from conduit 36 in advance ofblower 31. Whatever practice is followed a sufiicient amount of heliumis lfed into the system to iiush out most of the air. Enough helium isthus used to `place the system under substantial positive pressuregreater than atmospheric to keep outside air from seeping into thesystem. A manometer pressure of 2 Water has worked well.

Since helium and air are readily mi-scible, no matter what precautionsare taken an appreciable amount of air will be in the system after theflushing operation; and since this residue of air and the helium containobjectionable amounts of oxygen and nitrogen, purifier I8 is placed inoperation. As shown in Fig. l circulation of the helium through thesystem may be effected by the use of compressor I 3, as well as byblower 31, or both. In any event helium from the system is passedcontinuously and cyclically through the purifier. Heating gases, such asprovided by oil or gas burners, are fed through inlet 2B into heatingchamber I9, while spent heating gases escape from the heating chamberthrough outlet 2| to the open atmosphere. Impure helium from the systemis forced by compressor I3 through conduit I4, filter I5, conduit Io andbranch conduit I 1 into the purifier I3. The purified helium risesupwardly through conduit 22 and is returned to the main system.

As more particularly shown in Figs. 2, 3 and 4:, the impure heliumpasses upwardly through layers 35 of nely divided metal particles insuperposed perforated trays 82. The heating gases applied externally tothe purifier are adapted to heat the metal particles to a temperaturesufiiciently high for the oxygen and nitrogen impurities to reacttherewith to form metal oxide and metal nitride. Assuming that magnesiumpowder is to be produced, the layers are formed preferably of magnesiumpowder, about IAL" to 1/2 deep; the depth being such that the helium maypass readily therethrough. The oxygen and nitrogen impurities react withthe magnesium to form magnesium oxide and magnesium nitride which areretained in the layers on the trays. In the particular arrangementshown, the helium thus purified (see Fig. l) passes through conduit 22into gasometer I where it mingles with helium stil1 to be purified.

Since the circulatory system is open, helium passes continuously throughgasometer I, conduits 2, 3 and 4, filter 5, conduit 5, branch conduit 1,filter 8, branch conduit 3, branch conduit I, filter II, branch conduitI2, compres-- sor I3, conduit I4, filter I5, conduit I6, lbranch conduitI1, purifier I8, conduit 22, and again gasometer I. As this cyclicoperation continues, helium is also forced by blower 31 through conduit38, powder-gas separator 40, conduit 4I, powder-gas separator 42,conduit 43, filter 44, main conduit 3, branch conduit 45, filter 4B,conduit 41, as well as branch conduit 50, filter 5 I, conduit 52, mainconduit 3, branch conduits 3D and 33 into and through chamber 32, inlets|98, side compartments I 42 and I 43, outlets |93, conduit 38, and backto blower 31.

In a similar manner compressor I3 also pulls some of the helium frommain conduit 3 through branch conduit 4, dust lter 5, conduit B, branchconduit 1, lter 8, branch conduit 9, as well as branch conduit IIJ,filter II, branch conduit I2, conduit 6, the compressor itself, conduitI4, filter I 5, conduit I6 past branch conduit I1 to and through branchconduit 60, branch conduits 6I and 62 to furnace 64. As more clearlyshown in Figs. 5 and 6, helium passing through conduit 6I goes intomelting pot Ill. This helium may be passed through conduit 63 intochamber 32. The helium passing through branch conduit 52 moves throughthe coils surrounding the melting pot and is passed into and throughatomizer IIS, after which it mingles with helium passing through chamber32.

If furnace 14 is also placed in operating position with respect tochamber 32, its melting pot may .be flushed of air and filled withhelium undergoing purificati-on in a manner similar to furnace 64.Helium is forced by the compressor through conduits I6 and 1I), andbranch conduits 1I and 12. Helium passing through conduit 1I into themelting pot may be passed through conduit 13 into chamber 32. Heliumpassing through branch conduit 12 is passed through the coils around themelting pot and finally through the atomizer into chamber 32.

It will be clear from what has been said that if the impure helium inthe system is circulated for a sufficiently long time, it will continueto pass through the purifier until substantially all of theobjectionable oxygen and nitrogen are removed. Periodic tests are made,with an Orset tester, to determine the oxygen content of the helium.When it is sufficiently pure the valves in inlet I1 and outlet 22 areclosed to cut-out the purifier circuit. It may be cut-in from time totime as needed. If all goes well, the purified helium may be retained inthe system and used over a prolonged period of time.

With the circulatory system full of purified helium, it is ready for theactual metal powder producing operation. Referring for the moment toFig. 5, which shows melting furnace S4 and its auxiliary equipmentv in'more-f detail; cover.' 10i is removed from `char-ging. corlilui't' v1%; Ingots' off metal to be converted' into metaly powder" are droppedinto melting pot/lull; Arter. a 'suitable amount is placed therein thecover ifs'returned;k The apparatus is at present .beingu'sed't'o producemagnesium powder andalso magnesium-aluminum alloy powder. The insidedimensionsfof the pot are 24" diameter and@A Af charge' of aboutv 180Vpounds-ol magnesiumiszthusxintroduced. Oil burnersfl are operatedstoheat,` the pot externally. untilits magnesiumcontent reaches a suitablemolten temperature., fromv .1800?" to 1350 F. The burners-arekept inoperation to maintain this temperature;

Pressure-gas conduit 61| is; openedi to admitl helium into the chargeconduit-and'.topa-portion of the pot above the-level cfrthemcltenzmagfnesium. Since the pressureegas. conduit conv municateswith the compressor/the.mcltenzrnage nesium is placed undersufcientgas-ipressurel to: force a stream of magnesiumupwardly throughvmolten-metal conduit Mil to. andthrough atornizerV IB into atomizing andpowder: rormingzchamber 32. A pressure or JBZ-l5 poundsoper-squa'refinchis applied initially to 'the top of; the body of molten magnesium tostart. itszipassage through; the conduit and the atomizen' Afteratomization of the .metal .is underway, theipressurc is reduced to'about 5 pounds '.persquare inch. Helium under *adequateA pressure-.bythecompressoris passed throughV atomizingegasr conduit'` 62 ivi/here itis. heated in the coils around the meltingpot. The: preheated heliumispassed.y throught nipple- |3= (see Fig. 6) into gas .distributingchairiberv llt-Scofthe atomizer, from which it issues through dischargeports |32 of cap '|2S'in. a .plurality of streams or blasts and'atomizes thene stream of molten magnesium discharged through nozzleoutlet |23 at point or areailt a suitable-distance. forward of thenozzle tip. The pressure of the helium at the nozzlevaries somewhataccording to its construction. A pressure in. the neighborhood of 60pounds per square inch Works satisfactorily with the nozzles nowemplcyecl.`

Returning for a moment to-Fig.'1, it' will be recalled that blower 3lcauses the helium'to circulate continuously through the syste-m. Sincechamber 32 is on the suction-sideor the-blower, the helium is inxeffectsuckedthrough and from the chamber only to be forced or pushed: from thepressure side of the blower around the system. Relatively cool heliumenters the chamber by" way of branch inlet conduits 36 and: lat the sameend, adjacent melting furnace Sil- 'l-her atomized magnesium ispromptlyenvfelopedand frozen into powder particles in the cool heliumAsweeping through upper comp-artm'entI HN! (see: Figs. 7 and 8).

While the newly formed particles of magnesium powder tend to remain invSuspension. in: the helium as the helium.. passes through thecham.- berto the blower, some of it also; tenclsftcf settle byfgravityonto'plates|51 to lxand'towardthe bottom 'or base of theV trouglioftherupper.corn-Y partlnent. Additional'am'ountso relatiifelyncoel.helium are, therefore, introducedintonthezupper; compartment by way of.inletlconduit; Ac ccrding 'to a pre-'sent practice, the amount of heliumentering the upper rcompartment `by way;l of inlet S'San'd lowercompartment; 'orfductaV Mtl: substantially exceeds Vthat enteringloywvayfor :inilet conduit Sil; Some ofl the h-eliumrom inlet.: 33 issues as arelatively Vsmallstream;throng- 11; slot I 'l 5T,- lthe Is'tre'ain being'directed aiong.' theatopf oft plate |681T in', the-bottom of thetrough. Airyr magnesium;l powder settling or tending tcy settle onthat'platevis, therefore, sweptw up and placed: in suspensionin.themain-andlarge currentl ofV gas or helium passing throughthe uppervcompartnfuerit;v

In a similar-manner some heliumin lowercomp'artment or ducty IM passesas a stream-through siotfmffaloirg. the top of' plate Il ati the bottomof the trpugh.- of thev upper ycompartanentf. Similanstrteains lofhelium'pass through slots |86 to ati the bottom of the troughl of theupper cornpartment arecontinuouslyswept with streams of heliumzunder.suirlc'ient'velocity to keep the powderfirom settling and. to keep it insuspension. additional; stream of helium passes through duct |92 intothe discharge endof the upper-compartment; Some or. the helium thusintroduced through the'` slots alsoQspreads to the side and.'sweepsacrosspl'atesfl 5|`to |58 to keep themclean ofripowder; The'mixture of gas and powder is under-'a great deal- 'or turbulence whichinhibits settlin'gzor the powder. The powder laden gas 1- is inzeiTectsuckedv fromthe upper compartment throughout-,ietf 3ft-and blown throughconduit 33.

If melting furnimeiity must be shut down for some reason, such as forrepairs, melting furnace 'M (see Fig. 1) isput in operation at the otherend of. thechamber. In this case itis advisable to close inlet conduitSe anoto open 'inletwconduit 34. A'S before, anne stream orimolten.magnesium issuing from the nozzle ofv the atomizer is blasted with.helium 'and the atomized mag- 1. nesium` is promptly enveloped in therelatively coo1 helium gas entering the upper compartment by way of'inlet conduit 3|! as well as by helium entering the chamber throughinlet conduit 335.

'The force behind the line stream of molten metal issuingPv from thenozzle of the atomizer at either end or the chamber and the force of thehelium used. to atomize the stream. of molten, magnesium is sui'cient tothrow the Iatornized magnesium `'substantially across the `length of thechamber. tionthe directionuof the stream ofmolten.metal is inAgeneralconcurrent with the streamsV of'helium passed into and throughtheY chamber..` 0n the otherhand, when melting furnace M is in operationthe direction of the stream 'of molten metalissuing from the nozzle isin general lcountercurrent to the helium entering the upper compartmentby way of inlet conduit-Salani: concurrent: with the helium entering thechamber through inlet conduit 34. As a result of thesev contrarymovements, at least initially, the helium is kept in a turbulent state.In anyevent' the suction force of the blower is suiiicient to draw theresulting po'wderegas mixture from the chamber. i

Still referring te Fig. l, the magnesium powder laden helium is-orced'by blower 3l through conduit: 38 yto powder-gas separator 40;Asshown in Figs` 9, 10 and lflY and asdescribed in some de- 6 1 tailabove, the larger powder particles are sep arated sc'alectively from thehelium and smaller particles in cyclonel'iB-'andzfall into hopper 2&3.'After asufcient supply of .powder has collected in Vtloeuhopper,.some ofit withdrawn from time to time; care being taken not to break the powderseal'in 4discharge conduittiso that-air can enter the system.

Again returningrto' Fig. 1,7 the helium contain',-

ing .the smaller. particles of rnaegnesiunitpowder:is` forced. from.powderegas. separat-m'v @il through.

When melting furnace Al 4"' is in opera` conduit 4I to powder-gasseparator 42. Since the separator operates like the other, the sameprocedure is followed in withdrawing powder. As already indicated one ormore additional powdergas separators may be employed.

From the chamber to and through the powdergas separators the heliumundergoes a substantial drop in temperature. As it leaves the lastpowder-gas separator in the series itv still contains some fines whichshould be removed before it reaches the compressor. To this end thehelium leaving powder-gas separator 42 is passed through conduit 43 intodust-collector 44. In the present practice this is in the form of aplurality of filter bags, the specific device employed being a Dracoodust collector. While a substantial amount of the dust is thus removedfrom the helium, some remains.

Helium leaving dust-collector 44 passes into main conduit 3 and is thendiverted through branch conduit 45, filter 46 and conduit 41 back tomain conduit 3. This is a wet lter, of the oil type, which needs to becleaned from time to time. When cut-out of the circuit for that purpose,helium is diverted from main conduit 3 through branch conduit 58, asimilar filter 5I and conduit 52 back to main conduit 3.

A substantial amount of the helium thus treated passes continuouslythrough main conduit 3 and branch conduits 3Q, 33 and 34 back to chamber32. Some of the thus treated helium is, however, diverted from mainconduit 3 to the compressor. Additional steps are taken to removefurther amounts of dust from this helium before it reaches thecompressor. To this end the helium is diverted through conduit 4 intoand through filter 5, which may be an oil filter similar to filters 46and 5 I. Although the helium thus treated is substantially dust free, itusually contains some suspended oil and moisture, both of whichcontribute to the fire hazard. Helium leaving the lter enters conduit 6and is diverted through branch conduit l, filter 3 and conduit 9 back toconduit 6 which connects the inlet or suction side of compressor i3.When filter 8 is cut-out for cleaning, helium leaving filter 5 isdiverted from conduit 6 through branch conduit I0, filter I I, andconduit I2 back to conduit. Filters 8 and II are advantageously of thepot type containing a filter layer of steel wool or other suitablematerial, through which the helium is passed to abstract the suspendedoil and moisture.

Since the helium may pick up some oil and moisture in the compressor, itis again filtered. Helium leaving the outlet or pressure side of thecompressor is passed through conduit I4 and iilter I5. This lter maycontain a filter layer of felt, for example, such as a Cuno iilter,which abstracts the oil and moisture as the helium passes therethrough.The helium thus treated for the removal of powder, fines, oil andmoisture is passed through the remainder of the system, including thepurifier when cut-in, the melting pots, and the atomizers, as alreadydescribed.

Some losses of helium from the system are unavoidable, not only fromminor leaks but also when charging the melting furnaces, etc.Replacement helium must, therefore, be added to the system. However, bykeeing the system under positive pressure, ingress of air is for themost part avoided. Such oxygen and nitrogen as enter by replacementhelium is so small compared with the total volume of helium in thesystem that their effect is of little consequence.vv

16 They react with the atomized magnesium or highly heated powder in thechamber and are quickly eliminated. That is, after the system is inmetal powder producing operation, it acts to purify itself so far as theextremely small amounts of oxygen and nitrogen are concerned.

It will thus be seen that a conilned system may be provided in which aninert gas of high purity under positive pressure is continuouslycirculated while metal to be converted into powder is melted andatomized therein. Although the gas initially placed in the system is notsatisfactory for the purpose, it may be so treated as to remove harmfulimpurities. After the gas is puried, it may be used over a long periodof time to produce metal powder. While various inert or non-reactivegases may be employed, helium is especially suitable because of itsavailability. Metal powder may be produced from various metals and theiralloys, such as magnesium, aluminum, zinc, cadmium, lead, etc. Theprincipal limitation is the ability of the materials in the systemeconomically to withstand the necessary wear and tear.

It will be clear to those skilled in this art that the practice of theinvention lends itself readily to various modifications. The specicpractice described is only by way of illustration. 1t is obvious, forexample, that other compressors, puriers, furnaces, atomizers,separators, lters, etc., could be used and that the units making up thewhole can be arranged in various ways. What is disclosed is a highlyeffective arrangement for producing metal powders in large quantities inan eflicient manner.

I claim:

l. In the method of producing metal powder by atomizing molten metalwith a blast of inert gas, freezing the atomized metal into rielydivided solid particles, and recovering the resulting metal powder, theimprovement which comprises conducting the molten metal atomizing, metalpowder-forming and powder-separating steps in successive zones thereforof a main confined circulatory system lled with gas inert to the metal,circulating the gas continuously therethrough under a pressure higherthan atmospheric to prevent ingress of outside air, withdrawingcontinuously a relatively small portion of the inert gas from the maincirculatory system beyond the powder-separating zone and in advance ofthe atomizing and powder-forming zones, compressing the gas so Withdrawnsubstantially to increase its pressure above that at any point in themain circulatory system, and atomizing the molten metal in the maincirculatory system with at least a part of the compressed gas, thecompressed gas being returned to and mixed with the gas in the maincirculatory system.

2. Method according to claim 1, in which the circulating inert gas withthe newly formed metal powder suspended therein is passed from thepowder forming Zone through a plurality of powder-gas separating zones,metal powder is separated from the inert gas in each latter zone, andthe metal powder so separated is withdrawn from the system at eachlatter zone.

3. Method according to claim 1, in which the inert gas so withdrawn isspecially heated, a continuous ne stream of the molten metal is blastedcircumferentially with a plurality of streams of the heated inert gas ina metal powder forming zone lled with the main body of circulating inertgas, the main body of gas being sufcently low in temperature promptly tofreeze the atomized metal into nely divided particles, the main body ofcirculating inert gas with the newly yformed metal powder suspendedtherein is passed from the powder forming zone to a powder-gasseparating zone, metal powder is separated from the mai-n body of inertgas, and the metal powder so separated iswithdrawn from the system.

4. Method according to claim 1, in which the main body of circulatinginert gas with metal powder suspended therein is passed from the powderforming zone to a powder-gas separating zone, metal powder is separatedfrom the main body of inert gas by dry precipitation, and the metalpowder so separated is withdrawn from the system.

5. Method according to claim l, in which the main body of circulatinginert gas with metal powder suspended therein is passed from the powderforming zone to a powder-gas separating zone, metal powder is separatedfrom the main body of inert gas by wet precipitation, and the metalpowder so separated is withdrawn from the system.

6. Method according to claim 1, in which the main body of circulatinginert gas with metal powder suspended therein is passed from the powder`forming zone successively through a series of at least two powder gasseparating zones, coarser metal powder being separated from the mainbody of inert gas in an earlier zone by dry precipitations, and finermetal powder being separated from the main body of inert gas in a laterzone by wet precipitation.

'7. Method according to claim 1, in which sorne of the inert gastemporarily withdrawn from the system and compressed is heated beforebeing used to atomize the molten metal.

8. Method according to claim 1, in which some of the inert gas withdrawnfrom the system and compressed is conducted while under substantialpositive pressure to a conned space above a body of the molten metal tobe atomized and used to force a stream of the molten metal to theatomizing zone.

9. Method according to claim l, in which the gas in the system is puredwith respect to oxygen and nitrogen by passing it through a body of nelydivided particles of heated metal adapted to react with the oxygen andnitrogen to form metal oxide and nitride.

10. Method according to claim 1, in which gas in the system contaminatedwith oxygen and nitrogen is withdrawn vfrom the system, passed through abody of heated metal powder adapted to react with the oxygen andnitrogen, and the purified inert gas is returned to the system.

11. Method according to claim 1, in which gas in the system contaminatedwith oxygen and nitrogen is passed through a plurality of thin beds ofnely divided particles of heated metal adapted to react with the oxygenand nitrogen.

12. Method according to claim 1, in which gas initially in the systemand contaminated with an objectionable amount of oxygen and nitrogen, isfirst passed through a body of finely divided particles of heated metaladapted to react with the oxygen and nitrogen until the gas is puriedwith respect to the oxygen and nitrogen, and the atomizing, metal powderforming and powder separating steps are then conducted in the presenceof the inert gas so purified.

13. Method according to claim l, in which a fine stream of the moltenmetal is blasted with inert gas so `withdrawn in a metal powder' formingzone iilled with the main body of; circulate ing inert gas', the mainbody of circulating gas being' suificiently low in temperature promptly'to freeze the atoniized metai into finely divided soli-d particles, anda stream oi the inert gas withdrawn from the main body of gas isintroduced at. the bottom of the powder forming. zone to help keep thenewly forrriedfv particles of metal powder in suspension during theirpassagel to the powder separating zone.

14. Method according to claim 1, in which the inert gas so withdrawn isspecially heated, a iine stream of the molten metal is blasted with theheated inert gas in a metal powder forming zone filled with the mainbody of circulating inert gas, the circulating gas being sufficientlylow in temperature promptly to freeze the atomized metal into nelydivided particles, a plurality of streams of inert gas from the maincirculatory system is introduced at spaced intervals along the bottom ofthe powder forming `zone to help keep the newly formed particles ofmetal powder in suspension during their passage from the powder formingzone to the powder separating zone.

l5. Method according to claim l, in which a ne stream of the moltenmetal is blasted with the inert gas so withdrawn in one end of anelongated metal powder forming zone illed with the main body ofcirculating inert gas, the main body of circulating gas beingsufficiently low in temperature promptly to freeze the atomized metalinto finely divided particles, and a plurality of streams of inert gasfrom the main circulatory system is introduced along the bottom of thepowder forming zone to help keep the newly formed particles of metalpowder in suspension during their passage from the powder forming zoneto the powder separating zone.

16. Method according to claim l, in which the inert gas so withdrawn isspecially heated, a fine stream oi the molten metal is blastedcircumferentially with a plurality of ne streams of the heated inert gasso withdrawn in one end of an elongated metal powder forming zone filledwith the main body of circulating inert gas entering therein from thesame end, the main body of circulating gas being suihciently low intemperature promptly to freeze the atomized metal into nely dividedparticles, introducing the main body of circulating inert gas in thepowder forming zone at the same end so that the powder and gas may moveconcurrently through the powder forming zone, and a plurality of streamsof inert gas from the main circulatory system is introduced along thebottom of the powder forming zone to help keep the newly formedparticles of metal powder in suspension during their passage from thepowder forming zone to the powder separating zone.

17. Method according to claim 1, in which the gas so Withdrawn from themain circulatory system is filtered to remove solids therefrom before itis compressed.

18. Method according to claim l, in which a portion of the gas socompressed is used to force the molten metal into the atomizing zone.

19. Method according to claim 1, in which a part of the withdrawn andcompressed gas is puried with respect to oxygen and nitrogen by passingit through a body or" finely divided particles of heated metal adaptedto react with 19 the oxygen and nitrogen to form metal oxide andnitride.

20. Method according to claim 1, in which the gas Withdrawn from the'main circulatory system for compression is filtered to remove solidstherefrom before being compressed, and at least a part of the compressedltered gas is purified with respect to oxygen and nitrogen by passing itthrough a body of finely divided particles of heated metal adapted toreact with the oxygen and nitrogen to form oxide and nitride.

HENRY A. GOLWYNNE.

References Cited in the le of this patent UNITED STATES PATENTS NumberName l Date Nicol Sept. 7, 1920 Marx Jan. 18, 1927 Seastone July 26,1932` Paddle 1 June 18, 1946 Burkhardt Sept. 28, 1946

